The invention relates to integrated circuits that are sensitive to the risks of malfunctioning or destruction in the presence of electrostatic discharges, notably circuits made by MOS (metal-oxide-semiconductor) technology.
Electrostatic discharges can arise sometimes quite simply because a user touches the external terminals providing access to the integrated circuit chip. This risk may become very great with integrated circuits that are much handled. This is the case for example with circuits incorporated into chip cards: not only are the chip cards subjected to heavy handling but the access contacts are directly accessible without any mechanical protection or shielding device.
A known way of protecting integrated circuits against the risks of electrostatic discharge is to place electronic protection devices on the chip, each positioned in the immediate vicinity of a respective access pad. Each protection device is connected between an access pad and a ground pad (or more exceptionally between an access pad and another supply pad). The pads that are designed to receive an input signal at high input impedance are especially liable to undergo this arrangement.
FIG. 1 shows the general configuration of an integrated circuit chip, referenced IC, having a central part UC in which there is placed the active part of the circuit and a peripheral part surrounding the central part and having the access pads that enable the connection of the integrated circuit to the exterior. The pads are generally designed to receive soldered wires. Each pad is connected to the active part UC of the integrated circuit, to send it a voltage signal or a current signal or, on the contrary, to receive a signal therefrom. In FIG. 1, Pj designates one of these pads, connected by a conductor Cj to an input Ej of the active part of the integrated circuit. Hereinafter, the term "input Ej" shall be used to designate any point of a circuit that has to be connected to a pad Pj, even if, functionally, this "input" is actually an output of a signal to the exterior rather than an input of a signal coming from the exterior. P1 designates a ground pad normally held at a low potential Vss and P2 designates a supply pad giving a supply voltage Vcc. The pad P1 is generally connected to a conductive bus BM which conveys the ground potential all around the chip to bring it to various places of the circuit that need this potential. The same is generally the case with the supply pad at Vcc. However, no bus at Vcc has been shown in order to avoid burdening FIG. 1.
In a standard way, the devices for protection against electrostatic discharges are localized in the peripheral part of the integrated circuit, in the immediate vicinity of each pad to be protected.
The device used to protect a pad generally has a voltage limiter, connected between the pad to be protected and the common ground terminal. Its main function is to become suddenly conductive when the voltage at its terminals goes beyond a determined threshold. It thus shunts the energy of the electrostatic discharge received towards the ground. This voltage limiter naturally has a higher capacity of resistance to the energy received than the other elements of the integrated circuit, and it is furthermore designed to become conductive before the elements to be protected receive any harmful overvoltage.
FIG. 2 shows the simplest configuration generally used to protect a pad Pj connected by a conductor Cj to an input Ej of the active part of the integrated circuit. The input Ej is a point of a circuit element ELj which firstly must be connected to the pad Pj for the normal operation of the integrated circuit and, secondly, must be protected against electrostatic discharges. The element ELj is for example simply a transistor of the integrated circuit and, and the input Ej may be the gate of this transistor, incapable of withstanding excessively high voltages. It could also be a transistor source or drain acting as a signal input or output in a call exchanged with the exterior.
In the immediate vicinity of the pad Pj, in the peripheral zone of the integrated circuit, there is a limiter EC1j. It is connected between the pad Pj and the ground bus BM which passes near the pad.
The electrostatic discharges which are sufficient to activate the conduction of the limiter EC1j are diverted towards the ground bus BM, thus preventing overvoltages from appearing at the conductor Cj and hence at the input Ej. The arrangement is the same for the other pads to be protected with, for each pad, a limiter placed in the vicinity of the pad, connected between the pad and the ground bus BM.
The limiter element may be a simple diode whose anode is connected to the ground bus and whose cathode is connected to the pad to be protected (it assumed, as is almost always the case, that the ground terminal is the most negative supply terminal; otherwise the connections should be reversed).
The diode remains normally off while the voltage at the terminal to be protected does not go beyond an avalanche threshold. If the threshold is crossed, the diode becomes conductive and absorbs the current of the electrostatic discharge that has made it conductive. The limiter may also be a transistor.
It has been observed that this type of protection is not always satisfactory, inasmuch as malfunctioning or destruction could occur when electrostatic discharges are applied to the pads to be protected.
An aim of the present invention is to increase the effectiveness of electrostatic protection devices.
According to the invention, it is proposed to place another voltage limiter in the active part of the integrated circuit, in the immediate vicinity of a circuit element to be protected. This second limiter does not need to have a large capacity of current flow. It may be far more compact than the first limiter, so that it can be placed without any great inconvenience inside the integrated circuit in which there is located a circuit element (input transistor gate for example)to be protected particularly. Its trigger voltage is preferably substantially the same as that of the first limiter and its value is naturally chosen as a function of the voltage level that can be applied without damage to the circuit element to be protected.
The presence of this second limiter improves the protection in a way that shall be explained further below.
Consequently, the invention proposes an integrated circuit having an active central part surrounded by a peripheral part, the peripheral part having access pads connected respectively to circuit elements to be protected located in the active central part, with a voltage limiter device located in the vicinity of each pad in the peripheral part and connected between the pad and a common ground bus, wherein said integrated circuit comprises, for certain pads connected to a respective circuit element to be protected, a second voltage limiter placed in the central part of the integrated circuit, in the immediate vicinity of the circuit element to be protected.
A second limiter is not necessary for all the pads, as shall be seen: it is possible to use only one limiter for those circuit elements needing protection that are geographically quite close to the pad to which they are connected. The elements that are further away use a second limiter according to the invention. Furthermore, those circuit elements to be protected that are connected to pads which are themselves located sufficiently close to the pad to be protected could use only one limiter located in the peripheral part.
Preferably, a resistor is interposed between a respective pad and the circuit element corresponding to this pad and having to be protected. It limits the current directed towards this element and introduces a voltage drop between the pad and the second limiter as soon as a current flows in the second limiter.